Chapter 17 Cytoskeleton Essential Cell Biology FOURTH EDITION Copyright © Garland Science 2014 Alberts • Bray • Hopkin • Johnson • Lewis • Raff • Roberts • Walter
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-microtubules are hollow cylinders with an outer diameter of about 25 nm and an inner diameter of about 15 nm. - The wall of the microtubule consists of longitudinal arrays of protofilaments, usually 13 of them arranged side by side around the hollow center, called the lumen. - Each protofilament is a linear polymer of tubulin. Tubulin is a dimeric protein consisting of two similar but distinct polypeptide subunits, a-tubulin and b-tubulin. Microtubules
-They have a diameter of about 7 nm, which makes them the smallest of the major cytoskeletal components. -Microfilaments are polymers of the protein actin. Actin is synthesized as a monomer called G-actin (G for globular). - G-actin monomers polymerize into long strands of F-actin (F for filamentous), with each strand about 4 nm wide. - Each microfilament consists of a chain of actin monomers that are assembled into a filament with a helical appearance and a diameter of about 7 nm Microfilaments
-Intermediate filaments have a diameter of about 8–12 nm, larger than the diameter of microfilaments but smaller than that of microtubules. - The basic structural unit is a dimer of two intertwined, intermediate filament polypeptides. Two such dimers align laterally to form a tetrameric protofilament. Protofilaments then interact with each other to form an intermediate filament that is thought to be eight protofilaments thick at any point, with protofilaments probably joined end to end in an overlapping manner. Intermediate filaments
Bacteria Have Cytoskeletal Systems That Are Structurally Similar to Those in Eukaryotes (actin-like) (tubulin-like) (intermediate filament-like) a gram-positive, round- shaped bacterium a Gram-negative, oligotrophic bacterium 
INTERMEDIATE FILAMENTS
INTERMEDIATE FILAMENTS Intermediate Filaments Are Strong and Ropelike Intermediate Filaments Strengthen Cells Against Mechanical Stress The Nuclear Envelope Is Supported by a Meshwork of Intermediate Filaments
Figure 17–3 Intermediate filaments form a strong, durable network in the cytoplasm of the cell. Page 567
Pore complexes (TEM). Each pore is ringed by protein particles. Nuclear lamina (TEM). The netlike lamina lines the inner surface of the nuclear envelope. Nucleus Nucleus Nucleolus Chromatin Nuclear envelope: Inner membrane Outer membrane Nuclear pore Rough ER Pore complex Surface of nuclear envelope. TEM of a specimen prepared by a special technique known as f reeze-fracture . Close-up of nuclear envelope Ribosome 1 µm 1 µm 0.25 µm The Nucleus Page 567
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Desmosome structure They are especially abundant in skin, heart muscle, and the neck of the uterus Page 567
Figure 17–4 Intermediate filaments are like ropes made of long, twisted strands of protein. Page 568
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Structural Similarities of Intermediate Filament Protein 310-318 a.a . Page 568
Page 569 Figure 17–5 Intermediate filaments are divided into four major classes. These classes can include numerous subtypes. Humans, for example, have more than 50 keratin genes.
P disassembly *IFs differ greatly in amino acid composition from tissue to tissue ( intermediate filament typing) (connective tissue)
A mutant form of keratin makes skin more prone to blistering normal skin skin from mutant mouse The importance of this function is illustrated by the rare human genetic disease epidermolysis bullosa simplex , in which mutations in the keratin genes interfere with the formation of keratin filaments in the epidermis. Page 569 & 570
the plectin ( green ) links an intermediate filament ( blue ) to three microtubules ( red ) . The yellow dots are gold particles linked to antibodies that recognize plectin Page 570 Many of the intermediate filaments are further stabilized and reinforced by accessory proteins, such as plectin
Intermediate filaments support and strengthen the nuclear envelope Page 570 & 571
The disassembly and reassembly of the nuclear lamina are controlled by the phosphorylation and dephosphorylation of the lamins. Page 570 & 571
Children with progeria have wrinkled skin, lose their teeth and hair, and often develop severe cardiovascular disease by the time they reach their teens. Figure 17–9 Defects in a nuclear lamin can cause a rare class of premature aging disorders called progeria Page 571
1.Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm ? explain your answers. a. Amoeba proteus (a free-living amoeba) b. skin epithelial cell c. smooth muscle cell in the digestive tract d. Escherichia coli e. nerve cell in the spinal cord f . sperm cell g. Plant cell
MICROTUBULES
MICROTUBULES Microtubules Are Hollow Tubes with Structurally Distinct Ends The Centrosome Is the Major Microtubule- organizing Center in Animal Cells Growing Microtubules Display Dynamic Instability Dynamic Instability is Driven by GTP Hydrolysis Microtubule Dynamics Can be Modified by Drugs
MICROTUBULES
MICROTUBULES Microtubules Organize the Cell Interior Motor Proteins Drive Intracellular Transport Microtubules and Motor Proteins Position Organelles in the Cytoplasm Cilia and Flagella Contain Stable Microtubules Moved by Dynein
*Microtubules originate from microtubule-organizing centers (MTOC) within the cell. Fluorescence micrograph of a cytoplasmic array of microtubules in a cultured fibroblast Page 572
Each γ -tubulin ring complex serves as the starting point, or nucleation site , for the growth of one microtubule Page 573
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-tubulin: --- a minor species of tubulin --- complexes of -tubulin form ring structures that contain 10-13 - tubulin molecules (They serve to nucleate the assembly of new microtubules) Page 573
slow Assembly = disassembly fast Purified αβ-tubulin dimers at a high concentration can polymerize into microtubules spontaneously in vitro Page 574
Figure 17–13 Each microtubule grows and shrinks independently of its neighbors. The array of microtubules anchored in a centrosome is continually changing, as new microtubules grow ( red arrows ) and old microtubules shrink ( blue arrows ). Page 574
The selective stabilization of microtubules can polarize a cell nonpolarized cell Page 574
Figure 17–15 GTP hydrolysis controls the dynamic instability of microtubules. Dynamic Instability Model: involves a switch from growth to shrinkage or from shrinkage to growth of the microtubule or microfilament Page 575
GTP GTP -Tubulin-GTP -Tubulin- GDP Structure polarity and -tubulin dimer can each bind one GTP. The GTP in -tubulin is never hydrolyzed and is trapped by the interface between - and - subunits. By contrast, The GTP-binding site on -subunit is at the surface of the dimer, and can be hydrolyzed, and the resulting GDP can be exchanged for free GTP. Page 575
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2.Dynamic instability causes microtubules either to grow or to shrink rapidly. consider an individual microtubule that is in its shrinking phase. a. What must happen at the end of the microtubule in order for it to stop shrinking and to start growing again? b. How would a change in the tubulin concentration affect this switch? c. What would happen if only GDP, but no GTP, were present in the solution? d. What would happen if the solution contained an analog of GTP that cannot be hydrolyzed
Microtubule Dynamics Can be Modified by Drugs Page 575 & 576
The inactivation or destruction of the mitotic spindle eventually kills dividing cells. If a cell in mitosis is exposed to then drug colchicine , which binds tightly to free tubulin dimers and prevents their polymerization into microtubules, the mitotic spindle rapidly disappears, and the cell stalls in the middle of mitosis, unable to partition the chromosomes into two groups.
Figure 17–16 Microtubules guide the transport of organelles, vesicles, and macromolecules in both directions along a nerve cell axon. Page 576
Motor Proteins Drive Intracellular Transport Page 577 & 578
Both kinesins and dyneins move along microtubules using their globular heads Page 577 & 578
Different motor proteins transport different types of cargo along microtubules. Page 577 & 578
Microtubules help position organelles in a eukaryotic cell Page 579 Figure 17–20 Microtubules help position organelles in a eukaryotic cell. (A) Schematic diagram of a cell showing the typical arrangement of cytoplasmic microtubules ( dark green ) , endoplasmic reticulum ( blue ) , and Golgi apparatus ( yellow ). The nucleus is shown in brown, and the centrosome in light green. (B) One part of a cell in culture stained with antibodies to the endoplasmic reticulum ( blue, upper panel ) and to microtubules ( green, lower panel ). Kinesin motor proteins pull the endoplasmic reticulum outward along the microtubules. (C) A different cell in culture stained with antibodies to the Golgi apparatus ( yellow, upper panel ) and to microtubules ( green, lower panel ) . In this case, cytoplasmic dyneins pull the Golgi apparatus inward along the microtubules to its position near the centrosome, which is not visible but is located on the Golgi side of the nucleus. (B, courtesy of Mark Terasaki, Lan Bo Chen, and Keigi Fujiwara; C, courtesy of Viki Allan and Thomas Kreis.)
Outer microtubule doublet (a) A longitudinal section of a cilium shows micro- tubules running the length of the structure (TEM). (c) Basal body: The nine outer doublets of a cilium or flagellum extend into the basal body, where each doublet joins another microtubule to form a ring of nine triplets. Each triplet is connected to the next by non-tubulin proteins (blue). The two central microtubules terminate above the basal body (TEM). (b) A cross section through the cilium shows the ”9 + 2“ arrangement of microtubules (TEM). The outer micro- tubule doublets and the two central microtubules are held together by cross-linking proteins (purple in art), including the radial spokes. The doublets also have attached motor proteins, the dynein arms (red in art). Dynein arms Central microtubule Outer doublet cross-linking proteins Radial spoke Microtubules Plasma membrane Basal body 0.5 µm 0.1 µm 0.1 µm Cross section of basal body Triplet Ultrastructure of a Eukaryotic Flagellum or Cilium Plasma membrane Page 579 & 582
Microtubules in a cilium or flagellum are arranged in a “9 + 2” array Page 579, 582 & 583
FIGURE 16-8 Enlarged Views of an Axoneme. (a) This micrograph shows an axoneme from a flagellum of Chlamydomonas (TEM). (b) Diagram of an axoneme in cross section. The microtubules of the central pair have 13 protofilaments each, as do the A tubules of the outer doublets. Each B tubule has 11 protofilaments of its own and shares 5 protofilaments with the A tubule. The dynein sidearms have ATPase activity and are thought to be responsible for the sliding of adjacent doublets. The interdoublet links (nexin connections) join adjacent doublets, and the radial spokes project inward, terminating near projections that extend outward from the central pair of MTs. (c) Bending of the axoneme by dynein. Connection of the outer doublet MTs to the central pair converts sliding of adjacent MTs into local bending of the axoneme.
In humans, hereditary defects in ciliary dynein cause Kartagener’s syndrome . Men with this disorder are infertile because their sperm are nonmotile, and they have an increased susceptibility to bronchial infections because the cilia that line their respiratory tract are paralyzed and thus unable to clear bacteria and debris from the lungs. Page 583
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ACTIN FILAMENTS
ACTIN FILAMENTS Actin Filaments Are Thin and Flexible Actin and Tubulin Polymerize by Similar Mechanisms Many Proteins Bind to Actin and Modify Its Properties A Cortex Rich in Actin Filaments Underlies the Plasma Membrane of Most Eukaryotic Cells Cell Crawling Depends on Cortical Actin Actin Associates with Myosin to Form Contractile Structures Extracellular Signals Can Alter the Arrangement of Actin Filaments
Actin filaments allow animal cells to adopt a variety of shapes and perform a variety of functions Figure 17–28 Actin filaments allow animal cells to adopt a variety of shapes and perform a variety of functions. The actin filaments in four different structures are shown here in red : (A) microvilli; (B) contractile bundles in the cytoplasm; (C) fingerlike filopodia protruding from the leading edge of a moving cell;(D) contractile ring during cell division. Page 584
Microfilaments and Actin Structures Page 584
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64 Actin Polymerization in vitro Proceeds in Three Phases Addition of nuclei accelerates the rate Page 585
Actin Filament Treadmilling at Steady State
Actin Filament Treadmilling at Steady State Page 586
Treadmilling of actin filaments and dynamic instability of microtubules regulate polymer length in different ways Both microfilament and microtubule have treadmilling and dynamic instability Page 585 & 586
Figure 17–31 Treadmilling of actin filaments and dynamic instability of microtubules regulate polymer length in different ways. (A) Treadmilling occurs when ATP -actin adds to the plus end of an actin filament at the same time that ADPactin is lost from the minus end. When the rates of addition and loss are equal, the filament stays the same length—although individual actin monomers (three of which are numbered) move through the filament from the plus to the minus end. (B) In dynamic instability, GTP-tubulin adds to the plus end of a growing microtubule. As discussed earlier, when GTP-tubulin addition is faster than GTP hydrolysis, a GTP cap forms at that end; when the rate of addition slows, the GTP cap is lost, and the filament experiences catastrophic shrinkage via the loss of GDP-tubulin from the same end. The microtubule will shrink until the GTP cap is regained—or until the microtubule disappears (see Figure 17–15). Page 586
Treadmilling involves a simultaneous gain of monomers at the plus end of an actin filament and loss at the minus end: when the rates of addition and loss are equal, the filament remains the same size Page 586